Digital communication systems, such as modem systems, are well known in the prior art. Such systems typically employ timing recovery techniques that are utilized to recover the symbol rate at which the data is transmitted. Such systems also use synchronization techniques that are utilized to align the receiver clock with the transmitter clock. Synchronization signals are often transmitted near the beginning of a "handshaking" procedure, during a receiver training procedure, or periodically during data transmission (to resynchronize the receive modem with the transmit modem). The prior art is replete with various timing recovery and synchronization techniques; several timing recovery schemes are discussed in Lee & Messerschmitt, DIGITAL COMMUNICATION, pp. 737-764 (2d ed. 1996), the contents of which are incorporated herein by reference.
The current 56 kbps modem systems take advantage of the digital nature of the public switched telephone network (PSTN). The theoretical maximum data rate of 64 kbps per channel may not be realized in conventional 56 kbps modem systems that employ robbed bit signaling (RBS) to facilitate control signaling between network nodes. A particular data transmission may pass through a number of network nodes; each link may introduce RBS such that several bits are eventually robbed from the originally transmitted data codewords. The modem system can partially compensate for this type of digital impairment if it can identify the affected bits or the affected codewords.
For a given link in a 56 kbps modem transmission, RBS typically removes the least significant bit from the transmitted PCM codeword and replaces the bit with signaling information. The robbed bits are commonly forced to "ones" by the central offices, which effectively reduces the number of signal points in the signal point constellations utilized to decode the codewords affected by RBS. If the receiving modem can identify those symbols affected by the RBS, then different signal point constellations may be employed in a symbol-by-symbol basis to optimize performance and compensate for the RBS. Accordingly, a technique for synchronizing the receive modem to the RBS is needed to facilitate the application of signal point constellations to RBS-tainted symbols.
In current 56 kbps modem systems, RBS may occur in any given network link. In addition, symbols may be transmitted as a "continuous" stream or arranged in a number of signal segments or data frames having a particular number of symbols per segment or frame. The receive modem is configured to obtain and maintain synchronization with the transmit modem or network clock for purposes of timing and proper decoding. Present systems may utilize a special synchronization signal format or a symbol counting technique to monitor occurrences of RBS on a symbol-by-symbol basis. However, such techniques may break down if the receiver modem loses synchronization with the transmit modem. When such a receiver modem regains proper synchronization, it may not be possible to readily identify those symbols affected by RBS; the receiver modem may apply RBS-correcting signal point constellations where such correction is unnecessary. Untimely use of such corrective measures may introduce an undesirable number of decoding errors.
Therefore, a technique is needed to address the above disadvantages and shortcomings of prior art 56 kbps modem systems.